diff --git a/nixos/roles/monitoring/rules.yml b/nixos/roles/monitoring/rules.yml
index 8e70df55..6ec41b39 100644
--- a/nixos/roles/monitoring/rules.yml
+++ b/nixos/roles/monitoring/rules.yml
@@ -43,3 +43,9 @@ groups:
             severity: warning
           annotations:
             description: "mountpoint {{ $labels.mountpoint }} on {{ $labels.name }} out of inodes."
+        - alert: mailq
+          expr: postfix_showq_message_age_seconds_sum > 300
+          for: 5m
+          labels:
+             severity: warning
+             description: "mail queue {{ $labels.queue }} on {{ $labels.instance }} has accumulated a waiting time of {{ $value }}s.